1. Technical Field
The presently disclosed subject matter relates to a semiconductor light emitting apparatus which includes an array of a plurality of semiconductor light emitting devices. In particular, the presently disclosed subject matter relates to a semiconductor light emitting apparatus which employs color-converted semiconductor devices to produce a color mixture of primary light emitted by the semiconductor devices and secondary light which includes light that has been color-converted by a color-converting phosphor material or the like using the primary light as excitation light.
2. Description of the Related Art
Semiconductor light emitting apparatuses which employ semiconductor light emitting devices (hereinafter referred to as “LED chips” or simply “chips”) have been used for various types of illuminators such as headlights, streetlights, backlights, displays, ordinary lighting fixtures, etc. In order to produce light of a desired color, such as white light, semiconductor light emitting apparatuses often employ a combination of a light emitting device and a phosphor serving as a wavelength converting material. For such a semiconductor light emitting apparatus that utilizes color-converted semiconductor devices, the following methods may be employed to arrange a phosphor layer adjacent an array of a plurality of LED chips which are included in the light emitting apparatus.
As shown in FIG. 1A, one method includes applying a suspension containing a phosphor to an LED chip 122 mounted on a substrate 121, and to a contact electrode (not shown), and drying the suspension to thereby form a phosphor layer 123 in a uniform thickness around the LED chip 122. Such a method is disclosed, e.g., in Japanese Translation of PCT Patent Application No. 2003-526212 (corresponding Chinese Patent Application No. 01806034.X, European Patent application No. 2001919164, U.S. patent application Ser. No. 10/204,576). Another method includes forming a phosphor layer by screen printing using a metal mask that is patterned to the shape of a chip. This method is disclosed in Japanese Patent Application Laid-Open No. 2006-313886. According to this method, as shown in FIG. 1B, the phosphor layer 123 is deposited not only on the upper surface of a chip 122 but also between the chips 122. As shown in FIG. 1C, yet another method includes applying a resin dispersion containing a phosphor to individual LED chips 122, thereby forming a phosphor layer 123 only on the upper surface of the chips 122.
Another method other than the three methods mentioned above has also been suggested in which a phosphor layer is formed away from an array of a plurality of LED chips (see Japanese Patent Application Laid-Open No. 2004-288760).
In these conventional methods, a phosphor layer is directly formed on an LED chip as mentioned above. When an array of the plurality of LED chips is used, variations in chromaticity and luminance occur within an emission plane when viewed from above, in particular, in between the chips.
More specifically, when the individual chips 122 are coated with a phosphor layer as shown in FIG. 1A, emitted light is repeatedly reflected between the chips 122 and thus color-converted multiple times by the phosphor. This causes an increased amount of light component that is shifted toward the yellow wavelength area. Accordingly, when viewed from above the upper surface of the chip 122, a color shift towards yellow is observed in between the chips 122 with respect to the chromaticity immediately above the chips 122. To make matters worse, since no light emitting portions exist in between the chips 122, the non-light emitting portions appear as dark stripes in between the chips 122 when viewed from above the upper surface of the chips 122.
In the case in which the entirety of chips 122 are coated with a phosphor layer 123 as shown in FIG. 1B, a portion of the phosphor present in between the chips 122 may receive less excitation light than any other part thereof. In particular, the deeper (the closer to the substrate 121) in a particular portion of the phosphor, the less amount of excitation light reaches that particular portion. Accordingly, the phosphor can be considered to have a relatively higher concentration at that portion than at immediately above the chips 122, where a larger amount of excitation light is found. The apparent color provided by the light emitting apparatus depends on the color mixture of the color of light (excitation light) emitted by the chips 122 and the color of light emitted by the phosphor. Thus, when viewed from above the upper surface of the chips 122, a shift in color toward yellow is observed in between the chips 122 with respect to the apparent color exhibited immediately above the chips 122.
In the case when only the upper surface of the individual chips 122 are coated with the phosphor layer 123 as shown in FIG. 1C, a uniform color mixture may be obtained above the upper surface of the chips 122. However, due to the lack of emitted light components in between the chips 122, those non-light emitting portions in between the white-color light emitting chips 122 appear as dark stripes when viewed from above the upper surface of the chips 122.
When such a light emitting apparatus (LED package) is used as a light source for an optical system that is composed of a convex lens, a reflector and other components, variations in chromaticity and luminance may noticeably appear in the images projected by the system. To reduce these variations, a specific layer can be provided on top of the light emitting portion, with the layer containing a light diffusing agent, such as fine silica particles which can diffuse and scatter light. Nevertheless, this is not enough to eliminate those variations in chromaticity and luminance, but possibly raises another problem in that the light diffusion layer causes a decrease in luminous flux.
In contrast to this, the phosphor layer can be arranged over the upper surface of a plurality of LED chips with a gap between the chips and the layer. In this case, the aforementioned variations in chromaticity and luminance are significantly suppressed. However, this type of light emitting apparatus is difficult to reduce its light source in size. To facilitate optical designs, light sources should be desirably reduced in size.